Electrode array for field cages

ABSTRACT

An electrode configuration for field cages, especially in microsystems, comprises a large number of electrodes in which an electrical potential can be applied to each end region through a feed region. The end region is arranged to form the field cage and inhomogeneous shielding fields outside the field cage. To reduce thermal convection, the feed region has a strip form whose width is substantially smaller than characteristic dimensions of the end region.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/EP97/07002 which has an Internationalfiling date of Dec. 12, 1997 which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns electrode configurations for forming field cages,used especially in Microsystems for handling and manipulating particles,and field cage electrodes for use in such electrode configurations.

2. Description of the Related Art

To produce certain states or for the operation of certain processes innumerous biotechnical, medical, gene engineering and chemicalengineering systems, it is necessary to hold microscopic particlesprecisely in free suspensions or to move them in a predetermined manner.The particles of interest include biological cells or parts of them,latex particles or other socalled microbeads. By the influence ofelectrical fields in socalled field cages the particles are moved(“open” field cages) or held (“closed” field cages) in the suspension.In a held state the particles can be measured, for example, or caused tointeract with one another (cf U. Zimmermann et al. in“Electromanipulation of Cells” in CRC, 1996, chapter 5, pp 259-328).

The fields for influencing the particles are formed of electrodes,preferably fabricated by semiconductor technology methods. To form afield cage, a plurality of electrodes are configured either two- orthree-dimensionally and potentials are applied to them so that, in thespace enclosed by the electrodes (the socalled inner space), a fielddistribution is created in which a particle can be trapped or moved in aparticular direction.

The electrode shape and configurations for microsystem applications wereonly optimized to date in terms of the effectiveness of cage formationin the inner space (cf S. Fiedler et al. in “Microsystem Technology” 2,1-7, 1995). It is known, for example, that electrodes can be formed witha strip form tapering hyperbolically into the inner space for fieldcages of several micrometers to several hundred micrometers. Such simpleelectrodes were regarded to date as optimal in terms of field cageformation and their optimization through relatively simple computationof the field gradients. Nevertheless, the following effect is adisadvantage in conventional electrode configurations and shapes.

If the particle density in a liquid (suspension) is low, singleparticles, eg a cell, can be held stably in the field cage for a longperiod (minutes to hours). At higher particle densities however, furtherparticles enter the field cage in the course of time, which is generallyundesirable. This effect is initially surprising, because the field cagerepresents a potential barrier intended to prevent migration ofparticles into the inner space. But there are extra forces present,directed towards the inner space, that allow the particles to overcomethe potential. These forces are the result of thermal convection,especially warmup near the electrodes.

As a result of this effect, the use of field cages in microsystems isrestricted to relatively small particle densities, which are frequentlyunacceptable in practical terms.

SUMMARY OF THE INVENTION

The object of the invention is to present an improved electrodeconfiguration for cage formation and improved field cage electrodes forsuch an electrode configuration, with which larger particleconcentrations can be processed, with greater stability and reliability,than with conventional Microsystems.

This object is solved by an electrode configuration and a field cageelectrode of the present invention.

The invention is based on the idea of creating new electrode forms andconfigurations that, on the one hand, reduce or suppress thermalconvection flows and, on the other hand, enhance the effectiveness ofthe potential barrier surrounding the field cage. Electrodeconfigurations according to the invention consist of a large number offield cage electrodes, each with an end or head region to whichelectrical potential can be applied through a feed region. Unlikeconventional electrode forms with smooth, uniform edges, the end regionsof the electrodes in the invention are shaped so that highlyinhomogeneous fields are created in both the inner space (field cage)and the outer space. Here the inhomogeneity of the fields is so strong,or the field gradients so pronounced, that a shielding (or screening)field forms outside the field cage. The shielding field should be strongenough for forces to act on the particles that compensate for the forcesdirected inwards (eg through thermal convection). For this purpose theend region of an electrode exhibits electrode segments that, in part atleast, are limited by straight or slightly curved edges that abutagainst one another at predetermined angles. The edges abut against oneanother in such a way that an discontinous demarcation (formation of acorner or tip or the like) results. This means that high field lineconcentrations form on the edges of the end region, producing therequired, large field gradients.

In a preferred embodiment of the invention, in addition to the aboveshaping of the end region to create inhomogeneous shielding fields, thefeed region, through which the end region is connected to an electrodeterminal, has an electrode surface as small as possible. Preferably thefeed region is of strip form with a width optimized for the electricalpower. The formation of feed regions that are as narrow as possibleleads to a reduction of thermal convection.

The invention, unlike the electrode forms used to date and optimized forfield cage creation, produces instead electrodes whose end regions allowthe formation of field line concentrations, eg on edges or tips of theelectrodes.

Electrode configurations according to the invention can be arrangedplanar two-dimensionally, the holding or moving of particles then beingproduced by interaction of the field cage with parts of the microsystem(mechanical limiting). Alternatively the electrode configurations maytake on a three-dimensional form in which the particles are only held inthe field cages by the effect of the electrical forces. But in the caseof three-dimensional systems too, mechanical limiting can cooperate withthe field cages.

In a special embodiment of the invention, not only the end regions ofthe electrodes but also the feed regions are provided with electrodesegments that enable the formation of strong field gradients. Withsuitably designed interaction of adjacent field cage electrodes inparticular, this allows open and closed field cages to be combined, egin the form of a field cage with a feed channel.

Methods and devices according to the invention can be used incorrelation spectroscopy, especially for verifying fluorescent moleculeson the surface of submicrometer or micrometer particles and/or cells, orin pharmacological, medical diagnostic and/or evolutionary biotechnicalapplications. In particular, fluorescence correlation spectroscopy (WO94/16313) and other, especially confocal fluorescence techniques, asproposed in WO 96/13744 and European patent application 96116373.0, canbe used as verification methods. The last mentioned application suggestsa method for analyzing samples by repeated measurement of the number ofphotons per predetermined time interval of electromagnetic radiation,especially light, emitted, scattered and/or reflected by particles inthe sample, and determination of the distribution of the number ofphotons in the particular time intervals, whereby the distribution ofthe molecular intensities of the particles is found from thedistribution of the number of photons.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described below with reference tothe attached drawings. These show:

FIG. 1 shows a schematic plan view of a planar electrode configurationaccording to a first embodiment of the invention,

FIG. 2 shows a schematic perspective of an electrode configurationconsisting of two electrode configuration consisting of two electrodeconfiguration according to FIG. 1,

FIGS. 3A and 3B show a further embodiment of an electrode configurationaccording to the invention and field cage electrodes shaped according tothe invention,

FIGS. 4A and 4B show schematic plan views of electrode configurations inwhich open and clos3ed field cages according to the invention, arecombined, and

FIG. 5 shows a typical characteristic of field lines.

In what follows, embodiments of the invention are characterized indetail. A skilled person will see that features named in connection withcertain embodiments can be implemented in other embodiments. There is norestriction to particular combinations of electrode geometries orconfigurations.

DETAILED DESCRIPTION OF THE INVENTION

Detailed description of the invention is set forth in the followingdiscussion of the preferred embodiments. A skilled person will see thatfeatures named in connection with certain embodiments can be implementedin other embodiments. There is no restriction to particular combinationsof electrode geometries or configurations.

Depending on the application the electrode configuration shown can havea characteristic size in the region of 1 μm to 500 μm or larger. Theelectrode configuration is best created with planar semiconductortechnology on a chip and provided with suitable suspension feeds andsealing from its surroundings.

As an alternative to the planar electrode configuration shown in FIG. 1,it is also possible to create a three-dimensional field cage. This isachieved with an electrode configuration according to FIG. 2 consistingof two sub-electrode configurations 21 a, 21 b, each of whichessentially corresponding to the electrode configuration of FIG. 1.Between the sub-electrode configurations, created on a silicon or glasssubstrate for example, there is enclosed a suspension layer 22 with theparticles that are to be manipulated (trapped).

Further examples of electrodes embodying the invention are shown inFIGS. 3A and 3B. The planar electrode configuration according to FIG. 3Amatches for the most part the electrode configuration of FIG. 1, theelectrode end regions not being arrow-shaped however but taking the formof a semicircle. These semicircle segments forming the end regionsextend in directions that are perpendicular on a line connecting thecenter of the inner space and the electrode terminal. In other words, ifthe inner space is separated by reference planes from the particularelectrodes, the end region of each electrode extends in a direction on aparallel plane to the reference plane.

For effective retaining of particles from the outer space andsuppression of thermal convection flow, the spacing 31 between theelectrode terminal and the end region (ie the length of the feed region)should correspond at least to the spacing 33 between two oppositeelectrodes of the electrode configuration. Furthermore, it is preferredif the width 32 of the feed region is smaller than or equal to onequarter of the spacing 33. The electrode terminals, formed by feeders,present no restrictions in terms of width and length. It is preferredhowever, to reduce electrical losses, if the electrode terminals 35 areprovided with insulating layers. These should be between about 50 nm andseveral μm in thickness.

The narrow design of the feed region 35 presents the followingadvantage. A wide strip electrode means that a particle in the particlesuspension is exposed to a, for the most part, homogeneous electricalfield in which it can easily be shifted laterally, ie in the directionof the field cage too, by a suspension flow. This disadvantage of wideelectrode strips is overcome by the implementation of narrow electrodesegments according to the invention.

In addition to the feed region, the end regions of the field cageelectrodes can also be implemented with narrow strips, as exemplified byFIG. 3B in the case of the end regions 34 a-34 d (arrow shape, dualarrow shape, T profile, “antenna” shape).

The electrode configurations in FIG. 1 through FIG. 3 comprise betweenfour and eight identical electrodes. But it is also possible toimplement the invention with a different number of electrodes anddifferent electrode designs within one electrode configuration. A fieldcage configuration in a microsystem comprises in the widest sense(possibly in conjunction with mechanical means of limiting) at least onefield cage electrode with which a suspended particle is manipulated. Ifa plurality of electrodes are used, these are arranged as symmetricallyas possible in relation to the inner space in which the cage is formed.A square, regular multi-sided or circular arrangement is possible forexample. But asymmetrical arrangements are also conceivable.

FIGS. 4A and 4B illustrate microsystems comprising a combination of afield cage electrodes (FIG. 4A), or a sorting device 48, here meant tobe an open field cage, with a feed channel (FIG. 4B). The FIGS. 4A and4B each show one half of the respective microsystem. The entire systemis formed of a structural element with the illustrated electrodeconfiguration and a second structural element with an identicalcounter-electrode configuration as a mirror image. Both structuralelements are exactly aligned using spacers with the structured surfacesfacing one another. The particle suspension flows through the spacebetween the two structural elements. This space (ie the verticalinterval between the electrode configurations) is about as large as thefield cage 432 is wide.

According to FIG. 4A, two field cage electrodes of the field cage 43have a modified design of the feed regions 41 a and 41 b, whichthemselves form field cage electrodes for an open field cage. The feedregions form a feed channel that conducts particles in a predeterminedmanner along the path b to the field cage 43. But particles on the pathsa are conducted past the field cage 43 (extra mechanical means ofseparation, not shown, can be provided for this purpose). For controlledconducting of the particles to the field cage 43, the feed regions 41 aand 41 b are each provided with strip electrodes 44 directed inherringbone fashion to the middle of the feed channel to form electrodeend regions.

Conduction of the particles through the feed channel is obtained bypredetermined driving of the feed regions and the electrodes 41 a, 41 b,42 a, 42 b and the corresponding counter-electrodes (not shown) in theoverall system. All electrodes are driven by an appropriately selectedalternating potential of the frequency f, the following phase shiftbeing implemented for a specific particle conduction between theelectrodes (and corresponding counter-electrode Γ) 41 a: 0° Γ,: 180°, 41b: 180°; Γ: 0°; 42 a: 180°,: Γ; 42 b: 0° and: Γ180°. The potentialamplitudes should be between 0.1 and 100 V and can be selected by askilled person as a function of the particular application.

In FIG. 4B a feed channel, formed by the electrodes 45 a and 45 b,conducts particles along the path b to a sorting device 48 with theelectrodes 46 a and 46 b. Here the feed channel to the sorting deviceitself forms an open field cage in which the electrodes are providedwith electrode segments intended to produce strong fieldinhomogeneities. The electrode segments 47 a and 47 b are additionallyprovided to deflect particles along the paths a.

For the conduction of particles there are again four electrodes 45 a, 45b, 46 a, 46 b and the corresponding counter-electrodes (Γ, not shown).The path b is implemented by applying alternating potentials to theelectrodes 45 a, 45 b, 46 b, and leaving the electrode 46 a, floating,and likewise for the corresponding counter electrodes. The set phaserelationships are: 45 a: 0°,Γ: 180°; 45 b: 180°, Γ: 0°; 46 b: 0°, Γ:180°. If the particles are not to be conducted on the path b (shown) butpast the electrode 46 a, the electrode 46 b, would be floating and theelectrode 46 a, 180°. For corresponding counter-electrodes, thecorresponding counter-electrode for electrode 46 a would be 0°.

The invention also concerns a method for handling the describedelectrode configurations, especially for incorporating particles infield cages. The introduction of one or more particles for manipulationis characterized by the fact that a suspension flow is conducted throughthe electrode configuration while this is driven so that particlesconducted by the suspension are retained. In an electrode configurationaccording to FIG. 1 for example, the suspension is conducted in adirection running from bottom left to top right. Here the adjacentelectrode segments of the electrode end regions 17 d and 17 c, 17 a and17 b (possibly in conjunction with additional electrode segments for athree-dimensional configuration according to FIG. 2) form a channelthrough which the suspension is conducted. During this flow theelectrodes 13 a and 13 b are driven so that particles conducted by theflow come up against a potential barrier. At the same time theelectrodes 13 c and 13 d are switched off. As soon as a particle isdetected in the inner space 11, by an optical device for example, theelectrodes 13 c and 13 d are switched on again to close the field cageand shield any following particles. A special advantage of the electrodeconfiguration and the method according to the invention is that thisscreening of any following particles is very effective due to theimmediate formation of inhomogeneous shielding fields.

Typical drive protocols for the manipulation of living cells suspendedin physiological solutions and for 3 to 100 μm large beads (latex,sephadex, etc) are:

Cells (concrete: animal fibroblasts, 15 μm diameter) Drive amplitude 2.5to 8 V Useful frequency 1 MHz to 1 GHz (usually 5 or 10 MHz) Signalcharacteristic sinusoidal, squarewave Switching speed for cutting 1 usto 1 s electrodes in and out Duration of field application 1 ms to min

Functional ability of basic elements as function of flow velocity ofsolution in system:

Herringbone system (FIGS. 4A, 4B) up to 400 μm/s

Field cage (FIGS. 1, 2, 3) 50 to 100 μm/s

Diplexer (FIG. 4B, 46 a, 46 b) up to 1000 μm/s

Microbeads (concrete: latex, 15 μm diameter) Drive amplitude 2.5 to 15 VUseful frequency 10 kHz to 100 MHz (usually 100 kHz to 1 MHz) Signalcharacteristic sinusoidal, squarewave Switching speed for cutting 1 usto 1 s electrodes in and out Duration of field application 1 ms to min

Functional ability of basic elements as function of flow velocity ofsolution in system:

Herringbone system (FIGS. 4A, 4B) up to 2000 μm/s

Field cage (FIGS. 1, 2, 3) up to 80 μm/s

Diplexer (FIGS0. 4B, 46 a, 46 b) up to 2000 μm/s

The method exemplified above with reference to FIG. 1 can also beimplemented in other electrode configurations and geometries by the sameprinciple, the electrodes being driven in a two-step procedure so thatfirst a potential barrier is formed across the suspension flow and then,as soon as the particle or particles are in the inner space, the fieldcage is closed.

The electrode configurations and method according to the invention canbe used to measure, manipulate, concentrate, aggregate, immobilize,characterize or identify microparticles, cells, viruses, macromoleculesand other fluid or solid bodies of a size from 1 nm to 500 μm.

FIG. 5 shows an example of field lines in a field cage formed accordingto the invention on the basis of a modified arrow-shaped electrodeconfiguration. These are the equipotential lines (E²rms, averaged withtime), ie the potential for the dielectrophoretic force. Very noticeableis the asymmetrical field distribution in the cage and outside as wellas the basic arrow-shaped electrode form.

What is claimed is:
 1. Electrode configuration, comprising: a pluralityof electrodes surrounding an inner space which can be filled with asuspension and being arranged to create a field cage in the inner space,each of said electrodes having a feed region and an end region, whereineach feed region is arranged for applying an electrical potential to thecorresponding end region for creating field gradients in the innerspace; and strip-shaped electrode segments which are connected to eachend region of said electrodes, wherein said electrode segments form anarrow shape or a T-shape with rectangular extending strips with the endsof the electrode segments being directed opposite to the inner space andwherein said electrode segments are adapted to form inhomogeneousshielding fields outside the inner space that create field gradientsrepelling particles in the suspension away from the field cage in theinner space.
 2. Electrode configuration, comprising: a plurality ofelectrodes surrounding an inner space which can be filled with asuspension and being arranged to create a field cage in the inner space,each of said electrodes having a feed region and an end region, whereineach feed region is arranged for applying an electrical potential to thecorresponding end region for creating field gradients in the inner spaceand each end region comprises a semicircle section shaped electrodesegment at the tip of the end region; and strip-shaped electrodesegments which are connected to each end region of said electrodes,wherein said electrode segments form an arrow shape with the ends of theelectrode segments being directed away from the inner space and whereinsaid electrode segments are adapted to form inhomogeneous shieldingfields outside the field cage that create field gradients repellingparticles in the suspension away from the field cage.
 3. Electrodeconfiguration according to claim 1 or 2, wherein each feed region isformed by an electrode strip whose width is less than or equal to onequarter of a spacing between the electrodes in the inner space in whichthe field cage is formed.
 4. Electrode configuration according to claim1 or 2, wherein at least two electrodes are arranged so that the feedregions thereof form a feed channel-being capable to conduct particleson a predetermined path to the field cage.
 5. Electrode configurationaccording to claim 4, wherein the feed regions forming said feed channelcomprise a plurality of strip electrodes.
 6. Electrode configurationaccording to claim 5, wherein the plurality of strip electrodes form afishbone-like strip pattern pointing into the feed channel.